Widely available containers for drugs or drinks generally include aluminum cans, glass bottles and polyethylene terephthalate (PET) bottles. To the surface of such containers, a label was adhered with a pressure-sensitive adhesive at a strength that does not allow the label to be removed by hand. As Labels for beverage containers, wrap around labels (roll labels) made of polyethylene terephthalate (PET) film, biaxially oriented polypropylene film (OPP) or polylactic acid (PLA) film are often used.
In the recycle of containers with labels, it is necessary to send used containers to a factory and immerse the containers in a heated alkaline aqueous solution to separate the label from the containers. Therefore, pressure-sensitive adhesives applied to labels for containers are required to have a feature of swelling, softening, dispersing or dissolving in an alkaline aqueous solution, so that the label can be removed from the container in a short time (hereinafter, alkali-peeling property).
Japanese Patent Laid-Open Nos. 2005-220244, 2010-90185, 2010-280878, and 2012-1624 disclose hot melt pressure-sensitive adhesives which allow labels to be easily removed from PET bottles. The pressure-sensitive adhesives in the literatures are each an alkali-dispersible hot melt pressure-sensitive adhesive containing a styrene block copolymer and a tackifier resin.
Since the alkali-dispersible hot melt pressure-sensitive adhesives of Japanese Patent Laid-Open Nos. 2005-220244, 2010-90185, 2010-280878, and 2012-1624 contain a large amount of a petroleum-derived tackifier resin, the adhesives are not preferable in the light of environmental balance. Moreover, the alkali-dispersible hot melt pressure-sensitive adhesives of the literatures did not sufficiently meet the rigorous requirement of recent users for the adhesion to PET bottles.
In recent years, in the filed of a hot melt material, because of an increase in awareness of environmental problems, the replacement of conventional petroleum-derived raw materials by natural materials, plant materials and biodegradable materials has developed. In terms of environmental problems, such as global warming due to an increase of carbon dioxide emission, attempts have been made to produce hot melt compositions using non-petroleum-based resins, such as polylactic acid-based resins, containing no petroleum as a raw material.
Japanese Patent Laid-Open Nos. 2010-155951 and 2002-256250 describe hot melt compositions comprising a polylactic acid-based resin. Japanese Patent Laid-Open No. 2010-155951 discloses a hot melt composition comprising polylactic acid-based resin and poly(butylene succinate) or poly(ethylene succinate). Japanese Patent Laid-Open No. 2002-256250 discloses a biodegradable adhesive comprising polylactic acid and a natural powder substance. However, the hot melt compositions do not have sufficient adhesion to a substrate made of polyolefin and the like. Also, the hot melt pressure-sensitive adhesives comprising a polylactic acid-based resin described in the literatures do not provide good coating performance due to the poor compatibility between the polylactic acid-based resin and other components including tackifier resins. Therefore, the adhesives are likely to have lower adhesion and thermal stability than those of a hot melt pressure-sensitive adhesive in which no polylactic acid-based resin is used.